VirtualBox seems to lack the obvious "File -> Open" menu option. Frustrating!
I recently got a new workstation and I moved my VirtualBox and VMWare Player VMs from the old PC to the new.
The VMWare Player VMs are a snap... you go to "File -> Open a Virtual Machine...", find the .vmx files from the old PC, and voila! It's up and running. All the required files apparently are in the same folder.
With VirtualBox there just doesn't seem to be a way to do that. Looks like VMs are tied to the original host in some magical/invisible way like registry entries, etc.